First, to describe the construction of a liquid crystal panel used in, for example, a projector and display, FIG. 1 shows a sectional view of an example of the configuration of a liquid crystal panel. As shown in FIG. 1, liquid crystal panel 50 includes picture element electrodes 51 that are provided for each picture element and common electrode 52 that is provided opposite picture element electrodes 51, liquid crystal material 53 being injected between these electrodes. Light-shielding units 55 are provided between picture elements to block light and apertures 56 are provided in the portions of picture elements to transmit light. Apertures 56 are portions that can transmit light.
Although transistors for applying the voltage of picture signals are connected to each picture element electrode 51, these components are not shown in the figure.
Liquid crystal panel 50 shown in FIG. 1 is normally a black panel in which the picture element displays black when voltage is not applied across the two electrodes of picture element electrode 51 and common electrode 52. When the difference in potential between common electrode 52 and picture element electrode 51 is a minimum value or in the vicinity of a minimum value, the transmittance of light reaches a minimum value and the picture element displays black. When the difference in potential between common electrode 52 and picture element electrode 51 is a maximum value or in the vicinity of the maximum value, the transmittance of light reached a maximum value and the picture element displays white.
When black is displayed by a picture element, the voltage that is applied across the above-described electrodes is referred to as the “black side voltage,” and when white is displayed by a picture element, the voltage that is applied across the above-described electrodes is referred to as the “white side voltage.” When liquid crystal panel 50 is on the light path of green among the three primary colors of red, green, and blue, the display screen becomes green when the white side voltage is applied across the above-described electrodes. In the case of red and blue light paths, the display screen becomes red and green, respectively.
The operations of the liquid crystal molecules when a picture element displays white and black are next described. FIG. 2A and FIG. 2B are views for describing the operations of liquid crystal molecules of a liquid crystal material. FIG. 2A shows a case in which voltage is not applied across the two electrodes, and FIG. 2B shows a case in which voltage is applied across the two electrodes.
When voltage is applied to picture element electrode 51 with common electrode 52 as a standard, the liquid crystal molecule 60 of liquid crystal material 53 changes from the state shown in FIG. 2A to the state shown in FIG. 2B, and the orientation of liquid crystal molecule 60 becomes a fixed orientation. The orientation of the liquid crystal molecule is controlled by the field generated by the difference in potential between these electrodes (hereinbelow, this field is referred to as the “vertical field”) and light is polarized.
However, when picture elements are caused to display white, an electric field is produced by the difference in the potential of picture signals between adjacent picture elements, as shown in FIG. 1. This electric field is referred to as a “horizontal field.” When liquid crystal molecules 60 between picture elements receive the influence of a horizontal field and assume an orientation that differs from the ideal, phenomena such as light leakage are brought about.
The provision of light-shielding units 55 between picture elements prevents light leakage, but the aperture ratio, i.e., the ratio of apertures with respect to the area of one plane of a liquid crystal panel, has been increasing with the higher luminance, higher definition and smaller sizes of liquid crystal panels that are being used in recent years in, for example, liquid crystal projectors. As the aperture ratio increases, the area of light shielding decreases, rendering the configuration more susceptible to the occurrence of light leakage.
In the case of moving pictures in particular, picture element electrodes are charged and discharged by picture signals at a short cycle, and a particular image may cause liquid crystal molecules to orient in an abnormal direction under the influence of the horizontal field, giving rise to abnormalities such as the tailing phenomenon in the display image.
FIGS. 3A and 3B are images for describing an example of the tailing phenomenon of an image. Both figures show the state following movement by a triangle in the direction of the arrows in the figures. FIG. 3A shows the image when normal in which only the triangle after movement is displayed on the screen. FIG. 3B shows the image when an abnormality occurs in which, apart from the triangle after movement, an after-image appears on the screen that extends from the hypotenuse of the triangle in the direction of the triangle before movement.
This tailing phenomenon dissipates when the liquid crystal molecules return to their original direction, but because a time interval on the order of several msec to several tens of seconds elapses before the return to original orientation, the problems arise that, not only is the tailing phenomenon perceptible to the human eye, but in the case of a moving picture, the image also overlaps with the image that is displayed next. However, in a normally black panel such as liquid crystal panel 50, the tailing phenomenon dissipates instantaneously when a black image is introduced.
The above-described phenomenon in which an abnormality of a displayed image is brought about is known to occur when the display of a picture element of liquid crystal panel 50 changes from black to white. The phenomenon does not occur when the display of a picture element changes from white to black. Essentially, it is understood that abnormalities of a display image are related to differences in potential when an image changes and that the tailing phenomenon occurs when voltage is changed from a state of non-application of voltage across electrodes (or a state in which a small voltage is applied) to a state in the vicinity of the maximum voltage. The tailing phenomenon becomes more conspicuous the greater the difference of the voltage change. How to prevent the tailing phenomenon by reducing the difference of voltage change is under examination.
FIGS. 4A and 4B show waveforms of voltage that are applied to picture element electrodes. FIG. 4A shows an example of the normal voltage waveform, and FIG. 4B shows a waveform for a case in which the difference in voltage change is reduced from the waveform shown in FIG. 4A. The vertical axis indicates the voltage and the horizontal axis indicates time.
As shown in FIG. 4A, the applied voltage switches between the plus side and the minus side for each horizontal period. A picture element displays black at or in the vicinity of the minimum value of picture amplitude, and a picture element displays white at or in the vicinity of the maximum value of picture amplitude. Strictly speaking, a picture element displays white when the picture amplitude is 100% and a picture element displays black when the picture amplitude is 0% in the horizontal time intervals.
The time in which a picture element displays white in one horizontal time interval is determined by gradation information that is contained in the picture signal. In the example of the configuration shown in FIG. 1, voltages of plus 5V and minus 5V are alternately applied to picture element electrode 51 when the picture amplitude is 100%, and picture element electrode 51 is set to the same potential as common electrode 52 when the picture amplitude is 0%.
If the vertical axis of the voltage waveform shown in FIGS. 4A and 4B is assumed to be the picture level that corresponds to voltage, the picture level when a picture element displays white is referred to as the white level, and the picture level when a picture element displays black is referred to as the black level. The picture amplitude is a value that is proportional to the picture level.
In the voltage waveform shown in FIG. 4A, the danger of the occurrence of the tailing phenomenon arises because the voltage difference between white display and black display is 5V. In FIG. 4B, the voltage difference between the white level and black level is reduced from the case of the voltage waveform shown in FIG. 4A. More specifically, the absolute value of the voltage applied to picture element electrode 51 during the white level is 5V and thus is the same as for a normal case, but a voltage that is higher than the potential of the common electrode and lower than the white level is applied during the black level. In the example shown in FIG. 4B, the absolute value of the voltage that is applied to picture element electrode 51 during the black level is set to approximately 1V. The voltage value for raising the black level differs depending on the liquid crystal material or the construction of the liquid crystal panel.
An example of a method for reducing the tailing phenomenon is disclosed in JP-A-2008-046613 (hereinbelow referred to as Patent Document 1). According to the technology disclosed in this document, in order to reduce the tailing phenomenon caused by the influence of the horizontal field upon VA (Vertical Alignment) liquid crystal, the picture signal supplied to each picture element is checked and the picture signal is corrected by a correction table each time a predetermined voltage difference is surpassed.